Lubricants having improved load-bearing properties

ABSTRACT

The load-bearing capability of certain cyclic carbon-containing compounds particularly suited for use as a fluid component of tractive drives is substantially improved without impairing the oxidative stability or coefficient of traction of the fluid by incorporating minor amounts of zinc di(neoalkyl)phosphorodithioate.

United States Patent [191 Wygant Apr. 9, 1974 [54} LUBRICANTS HAVINGIMPROVED 3,440,894 4/l969 Hammann et al. 74 200 LOADBEARING PROPERTIES3.442.804 5/1969 LeSuer ct al 252/32.7 E

James C. Wygant, Creve Cocur, Mo.

Assignee: Monsanto Company, St. Louis, Mo.

Filed: Mar. 20, 1972 Appl. No: 236,437

, Related US. Application Data Continuation-impart of Ser. No. 26,446,April 7, 1970, abandoned.

Inventor:

US. Cl. 252/32.7 E, 252/59, 252/75 Int. Cl Cl0m 1/48 Field of Search252/32.7 E, 73, 75, 52 R,

References Cited UNITED STATES PATENTS 11/1968 Hammann et al. 74/200Primary ExaminerPatrick P. Garvin Assistant Examiner-Andrew H. MetzAttorney, Agent, or Firm-William H. Duffey 5 7 ABSTRACT 8 Claims, N0Drawings LUBRICANTS HAVING IMPROVED LOAD-BEARING PROPERTIESCROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of Ser. No. 26,446, filed Apr. 7, 1970, and nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to functional fluid compositions particularly adapted for use aslubricants in tractive drives, and more specifically, to additives forincreasing the load-carrying capability of these compositions.

2. Description of Prior Art A tractive drive is a device in which torqueis transmitted from an input element to an output element throughnominal point or line contact, typically with a rolling action, byvirtue of the traction between the contacting elements. While tractiveelements are commonly spoken of as being in contact, it is generallyaccepted that a fluid film is provided therebetween. Almost all tractivedrives require fluids to remove heat, to prevent wear at the contactsurfaces, and to lubricate the contacting elements, internal bearingsand other moving parts associated with the drive. The lubricating fluidbetween the contacting elements is subjected to extremely high stressand it is essential that the composition have a high load-carryingcapability in order to assure long life for the contacting elements.

Fluid compositions used in tractive drives are also useful as generallubricants for roller and ball bearings and other mechanical deviceshaving elements in rolling contact. Like tractive devices, theload-carrying capability of the lubricant is an important considerationfor many high stress applications. One particularly importantapplication is found in lubricating jet engine bearings where suchcompositions are effective to reduce bearing wear by reducing thepropensity of the bearings to skid.

It is accordingly an object of the present invention to providelubricating compositions having improved load-carrying capability andantiwear characteristics. It is a further object of this invention toprovide a class of additives to improve the load-carrying capability ofcertain compositions useful as tractive fluids without adverselyaffecting the tractive coefficient or the high temperature oxidationstability of the fluid. These and other objects of this invention willbe apparent from the ensuing description of the invention.

SUMMARY The load-carrying capability of certain lubricating base stockscomprising compounds having from to about 70 carbon atoms and includingat least two cyclohexyl groups is improved by incorporating into thebase stock a minor amount of zinc di(neo-alkyl)phosphorodithioatewherein the neo-alkyl groups have from 5 to about 13 carbon atoms andmay be the same or different. The addition of this material to thelubricant base stock composition significantly increases theloadcarrying capability of the composition as measured by standard weartests without significantly diminishing the high temperature oxidationstability as indicated by change in viscosity and corrosivity. Preferredadditives are zinc di(neo-hexyl)phosphorodithioate, zincdi(neo-pentyl)phosphorodithioate and mixtures thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS The compositions of the presentinvention which are generally useful as lubricants and particularlyuseful as lubricants for tractive devices are comprised of a majoramount of a base stock as hereinafter defined and a minor amount of azinc di(neo-alkyl)phosphorodithioate inhibitor.

The base stocks of the instant invention are comprised of a major amountof one or more cyclohexyl compounds having two or more cyclohexyl ringswhich are fused, concatenated, or linked by a C to C alkylene, acarboxy, or an ether linkage, and having a total of from 10 to aboutcarbon atoms. Preferred compounds are those having from 2 to about 6cyclohexyl rings, and more preferably from 2 to 4 such rings. A compoundhaving 3 or more cyclohexyl rings may have a structure which includescombinations of fused and/or concatenated and/or linked cyclohexylrings.

The cyclohexyl compounds are known to have good lubricating propertiesand relatively high coefficients of traction as disclosed in US. Pat.No. 3,440,894. In addition to such cyclohexyl compounds, the base stocksof the present lubricating compositions may contain minor amounts ofother lubricating compositions including, for example, paraffinic andnaphthenic petroleum oils, C -C polyolefins and synthetic lubricantssuch as monoand dicarboxylic acid esters. The utility and operability ofthe instant invention is in no way negated by the inclusion of minoramounts of such noncritical base stock constituents in combination withthe cyclohexyl compound which is the essential component.

As used herein the term major amount is defined to mean amounts greaterthan 50 percent by weight and the term minor amount" means amounts lessthan 50 percent by weight. While the compositions of this invention mustcontain at least a major amount of a cyclohexyl compound as hereinbeforedefined, it is generally preferred that the cyclohexyl compound bepresent in amounts of at least about 65 percent by weight and morepreferably at least about percent by weight of the total composition.

Examples of representative concatenated and linked cyclohexyl compoundsinclude dicyclohexyl, 4-(1- methylethyl )-dicyclohexyl, 4,4"bis(l-methylethyl dicyclohexyl, x-isohexyl-4'-isopropyldicyclohexyl,x-cyclopentyldicyclohexyl, dicyclohexylmethane,(xethylcyclohexyl)cyclohexylmethane, [x-cyclohexyl( lmethylethylcyclohexylmethane, bis( 2 ,4 ,6-trimethylcyclohexyl )methane, l l-dicyclohexylethane, l ,1 ,3-tricyclohexylpropane, l,2,3-tricyclohexylpropane, trimethylolpropane tricyclohexanecarboxylate,l ,2-tercyclohexyl, 1 ,3-tercyclohexyl, x'( l ,1- dimethylbutyl)-l,3-tercyclohexyl, x-( l ,1- dimethylbutyl)-l ,Z-tercyclohexyl, 1,2-isopropyltercyclohexyl, l,3-isopropyltercyclohexyl, bis( 1 ,3-cyclohexyloxy)cyclohexane, l,x-bis(methylcyclohexyl)cyclohexane,dicyclohexyl cyclohexane-l ,3- dicarboxylate, x,x'-quatercyclohexyl,tricylohexylmethane, bis-, cisand trans-1,2-cyclohexylcyclohexanedicarboxylate, l l -dicyclohexyl-2-methylpropane, 1l-dicyclohexyl-2-methylbutane, l l-dicyclohexyl- 2 ,S-dimethylhexane, ll -dicyclohexylpentane 1 ,2-dicyclohexylpropane, l,2-di(x-ethylcyclohexyl)- 3 propane, 2,2-dicyclohexylpropane,2,3-dicyclohexyl- 2,3-dimethylbutane, l,3-dicyclohexyl-2-methylbutane, l,B-dicyclohexylbutane and 2,4-dicyclohexyl-2- methylpentanc. Aparticularlypreferred class of such compounds are those selected fromthe group consisting of dicyclohexyl, alkyl dicyclohexyl, tercyclohexyl,alkyl tercyclohexyl, quatercyclohexyl, quinquicyclohexyl,2,3-dicyclohexyl-2,3-dimethylbutane, and mixtures thereof, wherein thealkyl contains from 1 to about 18 carbon atoms.

Examples of fused ring cyclohexyl compounds and compounds havingstructures including combinations of fused, concatenated, and/or linkedcyclohexyl groups include cisdecalin, trans-decalin, 2,3-dimethyldecalin, isopropyldecalin, t-butyldecalin, perhyclrofluorene,perhydrophenanthrene, perhydromethylcyclopentadiene (trimer),perhydrocyclopentadiene trimer, perhydrofluoranthene, 1- cyclohexyl-l,3,3-trimethylhydrindane, x-hexylperhydrofluoranthene,x-cyclohexylperhydrofluoranthene, poly(ethyll-methyl)perhydrolfluoranthene, xisopropylperhydrofluoranthene,perhydrofluorene-xcyclohexyl, perhydrofluorene-x-isododccyl,lcyclohexyldecalin, Z-(cyclohexyl-x-methyl)bicyclo(2,2,l)heptane,perhydropyrene, ethylperhydrofluorene, perhydroanthracene,bis-Z-decalin, lcyclohexyldecalin, 2-cyclohexyldecalin, dimethylcyclohexyldecalin, 4,5-methyleneperhydrophenanthrene,l,3-dicyclohexyloxycyclohexane, and cyclohexyl decahydronaphthyl ether.A particularly preferred class of such compounds are those selected fromthe group consisting of decalin, cyclohexyldecalin, alkyl substituteddecalin, alkyl substituted cyclohexyldecalin, and mixtures thereof,wherein the alkyl contains from 1 to about 18 carbon atoms.

In addition to the base stock materials and the zincdi(neo-alkyl)phosphorodithioate inhibitor, the compositions of thisinvention may contain other additive materials including for exampleviscosity index improvers, antioxidants, antiwear agents, corrosioninhibitors, dispersants, dyes, antifoam agents and the like.

Viscosity index improvers useful in the lubricating compositions of thepresent invention may be any of the polymeric materials commonlyemployed in the art. Particularly preferred are the polymers ofalkylesters of afi-unsaturated monocarboxylic acids, including forexample, poly(butylmethacrylate), poly(hexylmethacrylate),poly(octylacrylates), and poly(dodecylacrylates). Also useful arepolymers such as polyisobutylene, alkylated polystyrene, polyvinylethers, and copolymers of alkyl esters of monocarboxylic acids and othermonomers copolymerizable therewith as for example, the copolymer of analkyl mcthacrylate and vinyl pyrrolidone.

In a preferred embodiment of the present invention, the load-carryingand antiwear properties of the lubricating fluid are improved byincorporating into the fluid one or more zincdi-(neo-alkyl)phosphorodithioate compounds wherein the neo-alkyl groupscontain from 5 to about 13 carbon atoms and may be the same ordifferent, as for example zinc di(neo-pentyl)phos phorodithioate, zincdi(neo-hexyl)phosphorodithioate, or zinc(neo-pentyl)(neo-hexyl)phosphorodithioate. Although zincdialkylphosphorodithioates generally are known to be antiwear agents,these compounds as a class have been found to be unsuitable for use inthe base stocks of this invention because of an adverse effeet onoxidation resistance of the fluid. Examples of such known zincdialkylphosphorodithioate antiwear agents which were evaluated and foundto be unsuitable include those having as alkyl groups di( 1 ,3-dimethylbutyl), Z-ethylhexyl-isopropyl, Z-ethylhexylisobutyl,isobutyl-Z-ethylhexyl and di(n-hexyl).

In view of the consistently negative results obtained with many of thezinc dialkylphosphorodithioates on oxidation resistance of base stockmaterial, it was most surprising and unexpected to find that one classof compounds, the zinc di(neo-alkyl)phosphorodithioates of thisinvention, and particularly zinc di(neohexyl)phosphorodithioate,significantly improved the load-carrying capability of the fluid withoutdecreasing the oxidation resistance. The effectiveness of the additivewas especially surprising since many of the unsatisfactory compoundswere recognized antiwear agents generally useful in functional fluids.The n-alkyl equiv alent of the preferred additive of this invention,zinc di- (n-hexyl)phosphorodithioate, for example, reduced the oxidationresistance of the base stock as evidenced by a large increase inviscosity during a standard oxidation test. Likewise thel,2-dimethylbutyl compound caused a major increase in viscosity of thefluid in the oxidation test, and the isobutyl Z-ethylhexyl andZ-ethylhexyl isopropyl compounds each caused substantial increases incorrosivity of the fluid. On the basis of the unfavorable resultsobtained with these compositions, the excellent results obtained withthe di(neo-alkyl)additives of this invention were not to be predicted.

In accordance with this invention, zinc di(ne0alkyl)- phosphorodithioateis added to the base stock at a concentration of at least about 0.1percent by weight of the base stock, and preferably at a concentrationof from about 0.5 to about 2 percent by weight. Higher concentrationscan of course be used, but no substantial improvement in results isgenerally obtained thereby.

The nature and advantages to be gained by means of the present inventionare illustrated in the examples which follow. These examples arepresented for purposes of illustration only, and the invention is notintended to be limited to the specific embodiments presented therein.

In the following examples, the coefficients of traction was determinedon the Thrust Bearing Test Machine described in Effect of LubricantComposition on Friction as Measured With Thrust Ball Bearings by F. G.Rounds [1. Chem. and Eng. Data, Vol. 5, No. 4, pp. 499 (l960)]. Thismachine measures the torque transmitted from a central drive shaft to atorque arm through two thrust ball bearings which are submerged in thetest fluid. The bearings are shaft-mounted and can be rotated whilebeing subjected to an axial thrust load. Thrust loads are appliedhydraulically or by compressing calibrated Belleville springs. Atachometer geared to the drive shaft measures the rotational speed.Thermocouples located within fix-inch of the balls of the test bearingsmeasure the test fluid temperature which is held constant at variouspredetermined temperatures by heating or cooling the jacket fluid in thehousing surrounding the test chamber.

The individual balls tend to spin on an axis parallel to the principalbearing axis as well as roll around the raceway. As a result, bothrolling and sliding actions contribute to the traction. The outputtorque is measured with the torque arm which is fitted between the twobearings. This measured torque is then interpreted in terms ofcoefficient of traction for the tractant being evaluated. Thecoefficients obtained from this test machine are relatable to thosemeasured in actual tractive drives. Hence, the machine is effective forscreening candidate fluids. In determining the coefficients of 5 Thedata in Table II below illustrate the effectiveness traction reported inTables I and Il below, tractant tem of the preferred zincdi(neo-hexyl)phosphorodithioate perature was maintained at 200 F., theHertz stress additive in reducing the wear scar diameter when preswas500,000 psi, and the linear ball velocity was 800 ent in the base stockover a concentration range of feet per minute. from 0.1 to 1.0 percentby weight of the base stock.

The Oxidation and Corrosion test (0 & C) was con- The composition of thebase stock was essentially the ducted according to Federal Test MethodNo. same as that described for the tests shown in Table I. 791-5 308.4under the following special conditions:

Temperature 350F. TABLE II Time 72 hours Air 5 litcrs l5 Metals M8. Cu,Fe Concentration of Wear Scar Example Zinc di(nco-hcxyh- Diameter,Improvement The Wear Scar Test Data was obtained according to phmphmdmmmthe standard Shell 4-ball test, operated with 52100 steel 9 0 03/6 ballsat 200 F. and 1260 rpm. under a 40 kgm. load. I

The data in Table l below illustrates the advantages 0 of zincdi(neo-alkyl)phosphorodithioates over related 11 0.25 .49 35 compoundsin a typical tractant composition consistl2 ing of a mixture ofdicyclohexyl, tercyclohexyl and 2,3- dicyclohexyl-2,3-dimethylbutanecontaining 1.5 per- 13 1.0 .44 42 :S g i i ii i m pi'b i if pyrrohdonecopoly mprovement 0.76 Wear Scar Diameter/0.76

TABLE 1 Example Additive Coefficient O & C Test of Traction ViscosityCopper Corrosion"'" Wear Scar Test l None (Control) 0.0618 6 --0.04 0.622 1% di(neo-hexyl) 0.0638 13 +0.40 0.43 3 1% di(neo-pcntyl) 5 l.3 0.44 41% di(n-hexyl) 0.0629 2050 1.03 5 1.2% 1,3-dimethylbutyl 180 0.7 0.46 61% Z-ethylhexyl isopropyl 2344 3.5 7 1% isobutyl2-ethylhexyl 1800 7.70.49 8 2% 2ethylhexyl isopropyl 0.0623 24 -5.4

( l) Zinc (alkyl alkyl) phosphorodithioate, by weight of base fluid (2)Viscosity increase at I00F. Specification: less than (3) Mg. metallost/sq. cm. surface area Specification: less than O.6

(4) Scar diameter Specification: less than 0.45 mm.

The data of Table I clearly illustrate the advantages It is apparentfrom the above data that substantial imto be gained by using the zincdi(neo-alkyl)phosporodiprovement in the load-carrying capability of thefluid thioate additives of this invention. In Example 2, a preisobtained by the addition of as little as 0.1 percent of ferredembodiment of this invention, the addition of zincdi(neo-hexyl)phosphorodithioate. zinc di(neo-hexyl)phosphorodithioate tothe base fluid Although the preceding description and examples ofExample 1 decreased the wear scar diameter from have been directedprimarily toward the use of zinc 0.62 mm. to 0.43 mm. The small increasein viscosity di(ne0-pentyl)- and di(neo-hexyl)phosphorodithioate of 13percent during the O & C test was well within the as the additives ofthis invention, the neo-alkyl grounds specification set for thisproperty. The small weight may be different and may contain up to about13 cargain in the copper corrosion during the O & C test is bon atoms.Included, therefore, within the scope of the indicative of a protectivedeposit being formed on the Present invention are, for p Zinc metalsurface. Test results and visual observations indialky )p p t at ein thenee-alkyl group cate that this deposit does not interfere with the func-S neo-heptyl, neo-octyl, neo-decyl, neo-undecyl, neoi of h l b idodecyl, neo-tridecyl, and mixtures thereof.

A similar improvement i the wear Scar test d was The embodiments of thisinvention in which an excluobtained by adding 1 percent zincdi(neo-pentyl)phos- Sive P p y Privilege is Claimed are defined asphorodithioate to the composition of Example 1 as IOWSI shown by thedata in Example 3. In this case, however, A IUbTiCatIFIE CompositionConsisting essentially of a small increase in the copper corrosion rateduring the a base Stock Selected from group Consisting of O & C testaccompanied the addition of the additive. dicyclohexyl, alkyldicyclohexyli tercyclohexyl In certain applications where a corrosionrate of this alkyl tercyclohexyl, quatercyclohexyl, quinquicymagnitudeis not acceptable, it is contemplated that a clohexyl,2,3-dicyclohexyl-2,3-dimethylbutane and copper metal deactivator can beincluded in the tractmixtures thereof, wherein the alkyl contains fromant formulation.

The additives of Examples 4 and 5 were unsatisfactory because of thelarge viscosity increases occurring during the O & C test. Examples 6, 7and 8 show the effect of other known zinc alkylalkylphosphorodithioateload-carrying additives on the oxidation stability of the defined basestocks and further illustrate that zinc phosphorodithioate compounds asa class are not suitable additives for use in these compositions.

l to about 18 carbon atoms, and

B. an amount of at least about 0.] percent by weight of the base stockand sufficient to improve the load-carrying capability of the base stockof a zinc di(neo-alkyl) phosphorodithioate wherein the neoalkyl groupscontain from about to 13 carbon atoms and may be the same or different.

2. A lubricating composition consisting essentially of -A. a base stockselected from the group consisting of decalin, cyclohexyldecalin, alkylsubstituted decalin, alkyl substituted cyclohexyldecalin, and mixturesthereof, wherein the alkyl contains from 1 to about 18 carbon atoms, and

B an amount of at least about 0.1 percent by weight of the base stockand sufficient to improve the load-carrying capability of the base stockof a zinc di(neo-alkyl) phosphorodithioate wherein the neoalkyl groupscontain from about 5 to 13 carbon atoms and may be the same ordifferent.

3. A composition of claim 1 wherein the zinc di(neoalkyl)phosphorodithioate is selected from the group consisting ofzinc di(neo-pentyl)phosphorodithioate, zincdi(neo-hexyl)-phosphorodithioate and mixtures thereof.

4. A composition of claim 1 wherein the amount of the zincdi(neo-alkyl)phosphorodithioate is from about 0.5 to 2 percent by weightof the base stock.

5. A composition of claim 1 wherein the base stock comprises a mixtureof dicyclohexyl, tercyclohexyl and 2,3-dicyclohexyl-2,3-dimethylbutanev6. A composition of claim 5 wherein the zincdi-(neoalkyl)phosphorodithioate is zinc di(neo-hexyl)phosphorodithioate.

7. A composition of claim 2 wherein the zincdi-(neoalkyl)phosphorodithioate is selected from the group consisting ofzinc di(neo-pentyl)phosphorodithioate, zincdi(neo-hexyl)phosphorodithioate and mixtures thereof.

8. A composition of claim 2 wherein the amount of zinc di(neo-alkyl)phosphorodithioate is from about 0.5 to 2 percent by weight ofthe base stock.

2. A lubricating composition consisting essentially of A. a base stockselected from the group consisting of decalin, cyclohexyldecalin, alkylsubstituted decalin, alkyl substituted cyclohexyldecalin, and mixturesthereof, wherein the alkyl contains from 1 to about 18 carbon atoms, andB. an amount of at least about 0.1 percent by weight of the base stockand sufficient to improve the load-carrying capability of the base stockof a zinc di(neo-alkyl) phosphorodithioate wherein the neo-alkyl groupscontain from about 5 to 13 carbon atoms and may be the same ordifferent.
 3. A composition of claim 1 wherein the zincdi(neo-alkyl)phosphorodithioate is selected from the group consisting ofzinc di(neo-pentyl)phosphorodithioate, zincdi(neo-hexyl)-phosphorodithioate and mixtures thereof.
 4. A compositionof claim 1 wherein the amount of the zincdi(neo-alkyl)phosphorodithioate is from about 0.5 to 2 percent by weightof the base stock.
 5. A composition of claim 1 wherein the base stockcomprises a mixture of dicyclohexyl, tercyclohexyl and2,3-dicyclohexyl-2,3-dimethylbutane.
 6. A composition of claim 5 whereinthe zinc di-(neo-alkyl)phosphorodithioate is zincdi(neo-hexyl)phosphorodithioate.
 7. A composition of claim 2 wherein thezinc di-(neo-alkyl)phosphorodithioate is selected from the groupconsisting of zinc di(neo-pentyl)phosphorodithioate, zincdi(neo-hexyl)phosphorodithioate and mixtures thereof.
 8. A compositionof claim 2 wherein the amount of zinc di(neo-alkyl)phosphorodithioate isfrom about 0.5 to 2 percent by weight of the base stock.